Abstract
BackgroundAtelectasis is a major cause of hypoxemia after coronary artery bypass grafting (CABG) and is commonly ascribed to general anesthesia, high inspiratory oxygen concentration and cardiopulmonary bypass (CPB). The objective of this study was to evaluate the role of heart-induced pulmonary compression after CABG with CPB.MethodsSeventeen patients without pre-operative cardiac failure who were scheduled for coronary artery bypass graft underwent pre- and postoperative thoracic computed tomography. The cardiac mass, the pressure exerted on the lungs by the right and left heart and the fraction of collapsed lower lobe segments below and outside of the heart limits were evaluated on a computed tomography section 1 cm above the diaphragmatic cupola.ResultsIn the postoperative period, cardiac mass increased by 32% (117±31 g versus 155±35 g, p<0.001), leading to an increase in the pressure that was exerted on the lungs by the right (2.2±0.6 g.cm−2 versus 3.2±1.2 g.cm−2, p<0.05) and left heart (2.4±0.7 g.cm−2 versus 4.2±1.8 g.cm−2, p<0.001). The proportion of collapsed lung segments beneath the heart markedly increased [from 6.7% to 32.9% on the right side (p<0.001) and from 6.2% to 29% on the left side (p<0.001)], whereas the proportion of collapsed lung segments outside of the heart limits slightly increased [from 0.7% to 10.8% on the right side (p<0.001) and from 1.5% to 12.6% on the left side (p<0.001)].ConclusionThe pressure that is exerted by the heart on the lungs increased postoperatively and contributed to the collapse of subjacent pulmonary segments.
Highlights
Despite the fact that most of the mechanisms leading to intraoperative pulmonary collapse vanish when patients awake and resume spontaneous breathing, postoperative atelectasis and hypoxemia may persist for several days [5,14]
Large segments of the lower lobes are located beneath the heart [15], and previous reports have described the mechanical compression of the airways by an enlarged left atrium in neonates, infants and adult patients, leading to extended atelectasis. [16,17,18] radioisotopic ventilation–perfusion studies have demonstrated reduced lower lobe ventilation in patients with cardiomegaly [19,20], a phenomenon that is reversible with prone positioning
This study describes several original findings: (1) coronary artery bypass grafting (CABG) with cardiopulmonary bypass (CPB) induced a 5-fold increase in collapsed lung parenchyma, accounting for 38% and 42% of the preoperative masses of the right and left lungs, respectively; (2) concomitantly, gas volumes of the right and left lungs decreased by 32% and 28%, respectively, whereas tissue volumes increased by 47% and 59%; (3) following surgery, the masses of the right and left heart increased by 22% and 34%, respectively, significantly increasing the orthogonal pressure exerted on subjacent lower lobes; (4) the fractions of Overall, gas and tissue volumes and mass of right lower lobe segments below and outside of the heart limits measured on the computed tomography (CT) section located 1 cm above the diaphragm
Summary
Atelectasis is a highly prevalent pulmonary complication in patients undergoing cardiac surgery with cardiopulmonary bypass (CPB) and an important cause of postoperative hypoxemia. [1,2,3] Pulmonary collapse occurs early after the induction of anesthesia and persists for several days postoperatively. [4,5,6,7] Studies based on thoracic computed tomography (CT) have shown that pulmonary collapse is mainly distributed to the dependent regions close to the diaphragm and may encompass up to 35% of the overall lung parenchyma. [2,8,9,10] Some mechanisms have been proposed to explain the pulmonary collapse in patients undergoing cardiac surgery, such as the use of high inspiratory oxygen fraction, [11] cephalic displacement of the relaxed diaphragm compressing the caudal portions of the lower lobes, [12,13] surgical manipulation of pulmonary structures and depressurization of the respiratory system during CPB to enable better visualization of the surgical field. Atelectasis is a highly prevalent pulmonary complication in patients undergoing cardiac surgery with cardiopulmonary bypass (CPB) and an important cause of postoperative hypoxemia. [2,8,9,10] Some mechanisms have been proposed to explain the pulmonary collapse in patients undergoing cardiac surgery, such as the use of high inspiratory oxygen fraction, [11] cephalic displacement of the relaxed diaphragm compressing the caudal portions of the lower lobes, [12,13] surgical manipulation of pulmonary structures and depressurization of the respiratory system during CPB to enable better visualization of the surgical field. The objective of this study was to evaluate the contribution of the cardiac mass to postoperative atelectasis in patients undergoing coronary artery bypass grafting (CABG) with CPB.
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